Evolution of Sex
Evolution of Sex The dilemma
The origin and subsequent maintenance of sex and recombination is a phenomena not easily explained by Darwinian evolution. Evolutionary mechanisms such as natural selection are not able to reveal why organisms should abandon asexual reproduction in favor of more costly and inefficient sexual reproduction. In his book, The Masterpiece of Nature: The Evolution of Genetics and Sexuality, Graham Bell described the dilemma in the following manner:
Sex is the queen of problems in evolutionary biology. Perhaps no other natural phenomenon has aroused so much interest; certainly none has sowed as much confusion. The insights of Darwin and Mendel, which have illuminated so many mysteries, have so far failed to shed more than a dim and wavering light on the central mystery of sexuality, emphasizing its obscurity by its very isolation.
Evolution of Sex The move to sexual reproduction
Most single-celled organisms reproduce asexually. Asexual reproduction is the formation of new individuals from cells of only one parent, without gamete formation or fertilization by another member of the species. If life on earth is derived entirely from these single-celled creatures then why was this simple-yet-efficient method of asexual reproduction set aside in favor of sexual reproduction?
Why does sex exist? In his book Evolution: The Triumph of an Idea, Carl Zimmer concedes:
Sex is not only unnecessary, but it ought to be a recipe for evolutionary disaster. For one thing, it is an inefficient way to reproduce
And sex carries other costs as well
By all rights, any group of animals that evolves sexual reproduction should be promptly outcompeted by nonsexual ones. And yet sex reigns
Why is sex a success, despite all its disadvantages?
Besides the difficulties associated with the sheer rarity of beneficial mutations and their much-more frequent cousins, the harmful deleterious mutations, there is the added problem related to the two different types of cell division (mitosis and meiosis). During mitosis, all of the chromosomes are copied and passed on from the parent cell to the daughter cells. Conversely, meiosis occurs only in sex cells (i.e. sperm and eggs). During the latter type of replication, only half of the chromosomal material is copied and passed on to the subsequent generation. Meiosis results in the production of completely new combinations of the parental genes, all of them uniquely different genotypes. These, in turn, produce unique phenotypes, providing unlimited new material for the process of natural selection.
It is those very facts (that meiosis has allegedly evolved the ability to halve the chromosome count -- but only for gametes -- and that it can actually provide unlimited new material) which make the meiotic process so incredible. The mechanism of meiosis is critical for sexual reproduction. Yet meiotic sex is alleged to have evolved 520 million years ago. How could the bacteria that are supposed to be responsible for the evolution of sex have stabilized a billion years ago, and then 500 million years after the stabilization, mutate enough to evolve the painstaking process of meiosis?
As yet, evolutionary biologists have been unable to come up with a single adequate explanation as to how somatic cells reproduce by mitosis (thereby maintaining the species standard chromosome number in each cell), while gametes are produced by meiosis -- wherein that chromosome number is halved so that, at the union of the male and female gametes during reproduction, the standard number is reinstated.
Evolution of Sex Conclusion
The origin of sex remains a mystery for those committed to a purely materialistic view of reality -- not to mention the origin of the incredibly complex meiotic process that makes sex possible, or the intricate development of the embryo (which is itself an engineering marvel). At conception, the chromosomes inherited from the sperm are paired with the chromosomes inherited from the egg to give the new organism its full chromosomal complement. Naturalists would have us believe that undirected occurrences brought about this marvelously interdependent process of (1) halving the genetic information; (2) recombining it through sexual reproduction. Not only is such a sophisticated mechanism required for the production of a sperm or egg cell via meiosis, but another equally intricate process also joins the genetic information during fertilization in order to produce the zygote (which will later become the embryo). To believe that purely materialistic processes, governed by the laws of chance, could have produced such a mechanism stretches credulity beyond reasonable limits.